Vertebrate animals display sexually dimorphic behaviors that are regulated by steroid hormones, but the mechanisms by which hormones affect the nervous system, and the nervous system in turn controls these behaviors, are poorly understood. In the African clawed frog, Xenopus laevis, sexually dimorphic vocal behaviors have been well documented and attributed to differences in neural pattern generation. Sex- specific neuronal firing patterns and membrane properties have been found in laryngeal motor neurons. I propose to examine one of these sex differences: the presence of the hyperpolarization activated current (Ih) in male motor neurons, but not females, and its effect on vocal behavior. I hypothesize that Ih acts to regulate excitability in the motor nucleus and thus plays a key role in the recruitment of motor neurons and the production of male-specific vocal behavior. I will examine the effect of pharmacological blockade and enhancement of Ih on the firing patterns of single motor neurons and on the structure of population activity using both a controlled premotor stimulus and a fictive activity model of male vocal behavior. [unreadable] [unreadable] [unreadable]